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PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 86 



consideration. Much more material than I have examined would be 

 required to determine the extent of variation in form within species 

 of dog sharks, but the specimens seen do not demonstrate great varia- 

 tion in form when individuals of the same length and sex are compared. 

 Mustelus norrisi is most closely allied to M. lunulatus Jordan and 

 Gilbert but may be distinguished from it by the more posterior 

 position of the first dorsal. The specimens of M. lunulatus seen by me 

 have the inner or lower labial fold definitely longer than the outer, 

 while the specimens of M. norrisi usually have the outer labial fold 

 the longer. The well-developed, sharply pointed, lower caudal lobe 

 in adults of both M. norrisi and M. lunulatus (see fig. 55) distinguishes 



Figure 54.— Typical dermal denticles from the lateral surface of Mustelus mustelus (Linnaeus), showing 



short ridges. 



them from all other species of the genus. M. norrisi is much more 

 elongate and slender in form than i\/. canis, has teeth with higher 

 crowns, a more strongly arched jaw, and narrower fins. It may be 

 easily distinguished from ^lustelus mustelus (Liimaeus) by comparisons 

 of the dermal denticles, which, in that species, are not similar in 

 structure on the flat surfaces of the body. 



Mustelus mustelus probably is not normally, if ever, present in 

 American waters but is most closely allied to M. californicus Gill. In 

 these two species, specimens of all ages have strongly ridged denticles 

 along the middorsal line, some of which have six instead of the foiir 

 ridges characteristic of the genus as a whole. Away from the mid- 

 dorsal line, the ridges of the denticles become weak and do not reach 

 more than half the distance to the posterior apex (see fig. 54), the 



